1. Field of the Invention
The present invention relates to a laser irradiation apparatus that can be used for crystallizing a semiconductor film. In addition, the present invention relates to a laser irradiation method and a method for manufacturing a semiconductor device that use the laser irradiation apparatus.
2. Related Art
A thin film transistor formed using a poly-crystalline semiconductor film (a poly-crystalline TFT) is superior in mobility by two digits or more compared to a TFT formed using an amorphous semiconductor film. Therefore, the poly-crystalline TFT has an advantage that a pixel portion and a peripheral driver circuit of a semiconductor display device can be integrally formed over a same substrate.
The poly-crystalline semiconductor film can be formed over an inexpensive glass substrate by using a laser annealing method. Lasers are classified broadly into two types of a pulsed laser and a continuous wave laser according to the oscillation method. The output power per unit time of the pulsed laser typified by an excimer laser is approximately three to six digits higher than that of the continuous wave laser. Therefore, the laser irradiation can be performed efficiently to the semiconductor film by shaping a beam spot (the region on the surface of the processing object where the laser beam is irradiated in fact) of a pulsed laser beam into an ellipse, a rectangle having a length of several cm on a side, or a line having a length of 100 mm or more through an optical system. That is to say, the pulsed laser has an advantage of high throughput.
In particular, even though the power of the laser beam fluctuates by several % between pulses of the pulsed laser beam, it is possible to prevent the crystallinity of the semiconductor film from varying due to the fluctuation of the power by using a square beam spot shaped so as to cover the whole pixel portion in the semiconductor display device. With the beam spot shaped thus, moreover, a portion of the semiconductor film having the inferior crystallinity formed by the edge portion of the beam spot can be prevented from forming in the pixel portion. Therefore, the crystallization can be performed uniformly by using the beam spot having the size large enough to cover the whole pixel portion. When the poly-crystalline semiconductor film obtained by this uniform crystallization is used as an active layer of TFT, the variation in the characteristic of TFT such as on-current and mobility can be suppressed.
However, it is considered that the laser crystallization can be performed uniformly by using such a square beam spot covering a comparatively wide range when the energy distribution of the laser beam is homogeneous. For example, in the case of the excimer laser, actually the energy distribution of the laser beam that is considered to be caused by the laser oscillator exists in the range of approximately 1% to 5% at the peak-to-valley value. The energy distribution of the laser beam causes the variation in the crystallinity of the semiconductor film. Moreover, the energy distribution is almost constant between pulses of the pulsed laser beam. Therefore, when the crystallization is performed by irradiating a plurality of pulses of a pulsed laser beam to the same region for the purpose of further enhancing the crystallinity, the variation in the crystallinity due to the energy distribution of the laser beam is amplified, which interrupts the homogeneity of the crystallinity. It is noted that the term “energy distribution” herein used means the energy distribution in the beam spot formed on the irradiated surface.
It is noted that when a beam homogenizer including a lens array with a plurality of lenses equipped and the like is provided in the optical path of the laser beam, the energy distribution of the laser beam can be homogenized to some extent. However, the beam homogenizer has its limits to homogenize the energy distribution, and it is difficult to homogenize completely the energy distribution. The more the number of lenses included in the lens array per unit area increases, the more homogenized the energy distribution of the laser beam can be when thinking from the viewpoint of the geometrical optics. In this case, however, when the number of lenses per unit area increases, the variation in the energy density of the laser beam due to the interference of the beam between the lenses is also amplified, and this may cause the periodic interference stripe. In addition, the lens is miniaturized, thereby requiring higher accuracy. Since the highly accurate lens is so expensive that this lens is not preferable to be employed in the optical system.